<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <input type="text" value="手机">
</body>
<script>
    let input = document.querySelector("input");
    input.onfocus = function(){
        if(input.value==='手机') input.value = '';
        this.style.color = '#333';
    }
    input.onblur = function(){
        if(input.value==='') input.value = '手机';
        this.style.color = '#999';
    }
</script>
</html>